The invention relates generally to the field of cardiography and more particularly to the field of synchronized tomographic cardiography especially in ultrasound. The specific embodiment described below uses ultrasonic techniques and it is to the field of ultrasound that this application is primarily directed. However, researchers in the electronics of cardiography will recognize that the principles involved also apply to the fields of nuclear medicine and radiology; that is, the probing medium may be X-rays or gamma-rays, for example, instead of sound.
Few tools could be of greater value in the diagnosis of heart disease than a camera which could take a moving picture of the interior of the heart. However, heart structures cannot be directly differentiated by X-ray techniques, although indirect techniques using X-ray absorbing compounds in the esophagus and blood stream can indicate the general outline of the heart and its cavities. As a practical matter, however, doctors in the past have had to rely primarily on the stethoscope and electrocardiogram (ECG), and to some extent on the angiogram, because there were no direct observational tools available for cardiac diagnosis short of open heart surgery.
Sonar, developed after World War I, lead eventually to the use of sonar principles in nondestructive testing to reveal faults in materials, for example. Nondestructive testing in medicine is called noninvasive diagnosis, and this latter field borrowed the technology of sonar for its own purposes because it was found that the reflectivity of sound can discriminate between adjacent soft tissues.
Ultrasound is sound generated at a frequency above the range of human hearing (20,000 Hz). In medical diagnosis, frequencies around 2 mHz are typical. As in sonar a single transducer acts as both a sender and receiver of ultrasonic energy. The delay which occurs between the emission of pulsed sound and the return of the echo is a direct measure of the distance to the reflecting surface. Under water this means the distance to an energy ship; in the body it means the distance from the chest to the left ventricle of the heart, for example. In contrast, X-ray images reveal the transmissivity of the structures through which the X-rays pass. X-rays show bones well because bone tissue absorbs X-rays much more than soft tissue. On the other hand, ultrasonic systems normally use reflection like sonar although a transmission mode is of course feasible and is used in some circumstances.
Reflection of sound is caused by a difference in the acoustic impedance of adjacent tissues. Because blood and muscle tissues have different acoustic impedances at ultrasonic levels, ultrasound is reflected at the interface between a blood filled cavity and the muscle tissue which defines the cavity. Thus structures like the left ventricle and even the mitral valve of the heart can be detected by ultrasound while these structures do not yield sufficient contrast to differentiate them using x-rays.
The ultrasonic transducer functions like a directional radar antenna in that it defines a narrow beam within which it can transmit energy or receive reflected energy. Hence, in the reception mode sound energy in the vicinity is ignored unless it falls within the narrow beam. Typical beam widths are on the order of 10 millimeters, and typically the depth of the ultrasonic beam need go no farther than 20 centimeters.
The simplest display technique, and therefore the first kind used, for indicating the output of an ultrasonic transducer is to plot the echo output of the transducer versus time on an oscilloscope. If this amplitude mode (A-mode) display were used with a transducer that was properly oriented to intersect the left ventricle, two amplitude peaks corresponding approximately to the known distance to the left ventricle and having a rather large spacing at the end of diastole could be recognized as indicating the width of the left ventricle at that angle of intersection.
Another type of display is the brightness mode (B-mode) where the spacing between colinear dashes indicates the distance between reflecting surfaces. A slow vertical drift with time of the B-mode display results in a motion mode (M-mode). As the transducer is repeatedly pulsed, if the reflecting surfaces undergo periodic movement relative to the direction of the transducer beam, the motion mode will show a plurality of wavy lines.
Until recently the M-mode has been the primary display format for ultrasound cardiography (echocardiography). If the transducer beam intersects a few characteristic structures like the right ventricle, interventricular septum, and left ventricle, the spacing between the wavy lines and the relative motion of the reflecting surfaces which the wavy lines indicate provide dual clues to the identity of the structure being observed. In fact, the M-mode is so good at displaying certain heart structures and functions that it has already become an accepted technique for corroborative diagnosis of mitral stenosis, a valvular heart disease studied by ultrasound to determine the mobility of the mitral leaflet and the presence of calcification. Using the motion mode, medical researchers have also found it possible to study functional outflow tract obstruction and to make planimetric measurements of the left ventricle, for example. While the diagnostic potential of ultrasound remained untapped until recently, the role of ultrasound in cardiology is now rapidly expanding as particular heart diseases are correlated with characteristic echocardiographic abnormalities.
Although A-mode and particularly M-mode display formats have been extremely useful in providing clinical information in the past, there are advantages to a two-dimensional anatomical cardiac imaging system with the capability of demonstrating motion. A-mode and M-mode techniques represent complex waveforms which one can learn to relate to the structures under observation. A two-dimensional imaging technique which pictorially represents these structures draws more freely on the cardiologist's powers of intuitive recognition.
Two dimensional imaging of the heart encounters a specific problem with which A-mode and M-mode displays did not have to cope. Since the heart structures are always in motion, obtaining a two-dimensional image of the heart as if it were at rest requires either (1) scanning much faster than the heart moves or (2) scanning much more slowly with some type of selectivity (gating) in the gathering of information. The rapid scanning technique is exemplified by electronic transducer arrays and by automatic mechanical scanners which move very rapidly.
The other technique is a relatively new one based on stop action sampling in the nature of a stroboscope. An image can be built up one line at a time (i.e., offsetting the transducer beam each time) taking successive sections of data in successive heart beats, but timewise in exactly the same point within each heartbeat. For illustration, if a heart gave 60 beats per minute and a stroboscopic light flashed once a second, the heart would appear motionless because theoretically it would be caught in exactly the same point of the cycle each time the light flashed.
The use of the ECG as a synchronizing device to sample the output of an ultrasonic transducer at the same point within each successive heart cycle is known as the cardiac gate principle. It has been pointed out that gating can be used at different points (phases) of the heart cycle to demonstrate motion of the heart structure. One image of the heart can be formed at the end of diastole and another image of the heart can be formed at the end of systole using a cardiac gating signal which is shifted timewise from the diastole signal. The clinical usefulness of cardiac images obtained in this manner has been discussed in the following references: King, "Stop Motion Cardiac Imaging", Cardiac Ultrasound, Hipona et al ed., in press; Teichholz, "Echocardiography in Coronary Artery Diseases", Ibid; and Kikuchi, "Development and Present Aspects of Ultrasono-cardio-tomography," Ultrasonics in Medicine, DeVlieger et al ed., American Elsevier Publications, 1974, pp. 230-238.
Computer techniques have been reported in sorting and assembling multiple images per heart cycle using recorded data from several heart cycles; Waag et al, "Processing and Display of Cardiac Ultrasound Data", Proc. 26th Ann. Conf. on Engineering in Med. and Biol., 1973, p. 419; Waag, "Computerized Cine Ultrasound Cardiography", Cardiac Ultrasound, Hipona et al, ed., in press; Wixson et al, "Computer Acquisition and Processing of Left Ventricular Echocardiagrams", Proc. 19th Ann. Conf. of Am. Inst. of Ultrasound in Med., 1974 p. 46; and McSherry, "Ultrasonic Cardiac Imaging and Image Enhancement Technique", lecture paper delivered at 1974 IEEE Ultrasonics Symposium, Milwaukee, Wisconsin.
The Kikuchi article in particular provides a good backdrop for the present invention. One of the systems described in Kikuchi calls for using an oscilloscope to display the echo information from a number of cardiac cycles one at a time from the same point in each cycle. Between cycles of course the transducer beam is repositioned so that in the end the plurality of closely adjacent lines that the oscilloscope has displayed one at a time will represent a tomograph (cross-sectional slice) of the heart when photographed for cardiac examination. For illustrating motion, the Kikuchi article suggests filming photographs of successive images with a movie camera. The result is an animated simulation of the motion of the structures in view. The Kikuchi article also refers to a multiplex (multi-image) display technique in which the first image is triggered by the R-wave of the patient's ECG current and there is a preset uniform interval between each image.